Target for x-ray tubes

ABSTRACT

A target for x-ray tubes, which is comprised of separate elements mechanically connected together whereby the element which includes the x-ray generating focal area is of a selected material mechanically attached to at least one element of a second material by means which efficiently permits ready transfer of heat from the first element to the second while permitting free thermal expansion of the elements relative to one another.

United States Patent 1 Holland Mar. 5, 1974 TARGET FOR X-RAY TUBES 75 Inventor: William P. Holland, West Redding, ""T' Saalbach Conn. Assistant ExammerDarwln R. Hostetter Attorney, Agent, or, FirmHarold A. Murphy; Joseph [73] Assignee: The Machlett Laboratories, D. Pannone; John T. Meaney lncorpolrated, Springdale, Conn.

[22] Filed: Feb. 28, 1972 ABST T 21 Appl. No.: 230,053 [57] MC Related A n ti Dam A target for x-ray tubes, which is comprised of sepa- [63] Continuation of Ser No 42 375 June 1 1970 rate elements mechanically connected together aban d (16 whereby the element which includes the x-ray generating focal area is of a selected material mechanically 52 us. Cl 313/60, 313/55 313/330 attached to one element of a Second material 511 lm. Cl. n01 j 35/10 by means which efficiently Permits ready transfer [581 Field of Search 313/55 60 330 heat from the first element to the While ting free thermal expansion of the elements relative to [56] References Cited one'another' FOREIGN PATENTS OR APPLICATIONS 8 Claims 9 Draw Figures 707,070 5/1941 Germany; 313/60 Hg TARGET FOR X-RAY TUBES This application is a continuation of Ser. No. 42,375 filed June 1, 1970, now abandoned.

BACKGROUND OF THE INVENTION In the manufacture of targets for x-ray tubes, the portion of the target which is to be subjected to electron bombardment for the resultant production of x-rays is preferably made of a high atomic number material except in cases where characteristic radiation is required. However, it has been found that many problems exist when making the entire target of high atomic number material, due at least in part to the fact that during operation of the device the target will become seriously damaged through high thermal gradients causing severe mechanical stresses which result from bombardment by high energy electrons. This produces cracking, warping, and focal track disruption. For example, the temperature assumed by a conventional tungsten target at the focal spot may approach 3,400C and such heat may create hoop stresses which produce radial cracking resulting in mechanical failure, or warpage which alters the target angle and thereby changes the focal spot size.

Attempts to overcome these and other problems have been made by forming a target of a selected refractory base material having high termal capacity,

' such as molybdenum or graphite, for example. On this base material is deposted a layer of high atomic number material which has high melting point and low vapor pressure. This layer, which may be vapor deposited, flame sprayed, or brazed, may cover one side of the base or may cover the entire base surface. Such materials as rhenium, tungsten, or suitable alloys are deposited in the selected area or areas and are attached by a metallurgical bond to the base material.

These coated targets, however, have also been found to be unsatisfactory because of the extreme difficulty in obtaining good adhesion of the deposit to the base material. Differences in thermal expansion coefficients have caused much of the failure in devices of this character.

SUMMARY OF THE INVENTION terial such as molybdenum, titanium or graphite, for

example, which will meet vacuum tube processing reand contract relative to the other without destroying the efficient heat transfer relationship achieved by mechanically joining the parts together. It has been found that heat flow across the interface between the focal track area and the backing is surprisingly efficient with a mechanical interconnection, while a metallurgical The above and other objections to the prior art are subsequent operation of the device, and the base element comprises a suitable material which will rigidly retain the first element in its prelocated position but will also act as an efficient heat sink to carry heat away from the first element and to radiate such thermal'energy away from the target.

'The first element, in accordance with this invention may be any suitable refractory metal, such as tungsten or tungsten-rhenium alloy, for example, which has a high enough melting point and which generates x-rays in copious supply when bombarded by electrons in the known manner. The base element may be any suitable material which will support the first element and act as a heat sink to carry the heat away from the first-element and dissipate it through radiation. The base element, preferably, should be a relatively low density mabond is unsatisfactory, as pointed out above.

In one form of the invention, a focal area member of tungsten or tungsten rhenium alloy is mounted in intimate physical engagement upon a backing of graphite and held thereupon by bolts. In another form, the focal area member rests in a cavity in the face of the backing and is firmly held in place by spring means. In a preferred form of rotating anode, the focal area member comprises a ring of the selected material which is sandwiched between two discs or rings of the selected base material, and is held therein through pressure exerted by the threaded clamping means which serves to attach the anode in place on'a central rotatable shaft.

BRIEF DESCRIPTION OF THE DRAWINGS The above and other advantages of the invention will become apparent from the following description taken in connection with the accompanying drawings, wherein:

FIG 1 is an axial section through an x-ray tube of the rotating'anode type showing a target structured in ac cordance with this invention; 1

FIG. 2 is an elevational view of the target in the tube of FIG. 1;

FIG. 3 is an axial section through a target having means for preventing relative rotary motion between the target members;

FIG. 4 is an axial sectional view of a target illustrating an alternative means of applying pressure to the parts;

FIG. 8 is an enlarged fragmentary sectional view of a slightly modified target embodying the invention; and

FIG. 9 is a side elevational view partly in axial section of an x-ray tube with a stationary target embodying the invention.

DESCRIPTION OF THE PREFERRED EMBODIMENTS Referring to the drawings, there is shown in FIG. 1 an axial sectional view of an x-ray tube of the rotating anode type which embodies a dielectric envelope in which is supported an anode 12 and a cathode 14. The cathode 14 includes a supporting cylinder 16 one end of which is sealed to a reentrant end portion 18 of the envelope. On the inner end of cylinder 16 is mounted one end of a transversely extending angled support bracket 20, in the free end of which is located a cathode head 22. The cathode head 22 contains an electron-emitted filament (see FIG. 6) to which a suitable electrical potential is applied through leads 24 extending externally of the tube through cylinder 16.

The opposite end of the envelope 10 carries the anode 12 which includes a target 26 mounted on one end of a rotor shaft 28 extending from a rotor 30 rotatably located in a neck portion 32 of the envelope. The rotor carries a skirt 34 bolted thereto, and the assembly is adapted to rotate rapidly when the tube is mounted in suitable inductive means surrounding the neck 32 and when the inductive means is energized.

In accordance with this invention, the anode target assembly comprises a focal track member 36 in the form of a ring made of suitable high atomic number material, such refractory materials as tungsten or tungsten-rhenium being particularly suitable. The focal track member 36 produces x-rays when bombarded by electrons from the cathode 22 in the usual manner of x-ray generators. The exposed surface or track of the focal target member 36 is inclined so that x-rays will pass from the surface out of the tube through the side wall of the envelope.

The focal target of conventional x-ray tubes usually comprises the entire target 26 or is a metallurgically deposited coating upon a suitable backing of high thermal capacity material. For example, the entire target 26 may be made of tungsten, or a target backing of tungsten, graphite, molybdenum or the like may carry on its surface a focal target of a deposited or metallurgically bonded material such as tungsten or tungsten-rhenium alloy.

It has been found that solid targets of tungsten do not have satisfactory thermal characteristics and, when bombarded by high density electrons, become damaged by'the resulting severe mechanical stresses. It has also been found that a coating of target material upon the surface of a backing will not prove satisfactory As applied to a rotating anode tube as shown in FIG. 1, the focal target member comprises a ring 36 having its lower surface positioned upon a surface of a backing disc 38, which surface is shaped to mate with the adjacent surface of the target ring. The upper surface of the backing disc or ring 38 is recessed to receive the target disc 36 as shown. However, as will be noted, the outer periphery of the recess is of a slightly larger diameter than the outer periphery of the target disc 36 so that disc 36 may thermally expand without damaging the backing disc which has a lower coefficient of expansion. Likewise the upper surface of the target disc 36 is recessed except in the actual focal track area which is to be exposed to the cathode. In this way a second backing ring or dome 42 can be nested within the recess, with clearance being provided between the outer peripheral edge of the dome and the outer side wall of the recess in the disc.36 so as to permit expansion of the disc 36 without damage to the dome. With a nested structure of this type the target disc 36 engages the adjacent surfaces of the backing disc 38 and dome 42 throughout relatively expansive surface areas to achieve efficient conduction of heat from disc or ring 36 to the disc 38 and the dome 42 as is desired.

When mounting a target assembly 26 on its supporting anode structure, it is important to'insure that the two rings or discs 36 and 38 are at all times held in the required closely abutting relationship. Therefore, there must be some means provided for this purpose. In the example shown in FIG. 1, this is achieved by providing the rotor shaft 28 with an enlarged portion or collar 40, and backing ring 38 is mounted over the end of shaft and seated upon the collar. The lower surface of the backing ring may be suitably recessed as illustrated to receive the collar. Then the focal track ring 36 is slid down over the shaft into intimate engagement with the since the metallurgical bond between the target and backing will not withstand the stresses resulting from the thermal shock of the impinging electron beam. Furthermore, it is difficult to obtain a thermal expansion match with suitable backing materials over a full operating temperature range which may extend from room temperature to approximately 3,000C. Also, even ifa satisfactory metallurgical bond could be achieved, when graphite is used as the backing the weak bond between atom layers will not withstand the mechanical stresses involved.

In accordance with this invention, the focal target member 36 is made as a completely separate element or part which is physically located upon a separate and independent backing 38 which comprises in itself a selected suitable high thermal capacity, high thermal emissivity material. While the two elements 36 and 38 are entirely separate, it is essential that they be maintained in physical contact throughout an extended area so that heat may be efficiently conducted from the focal target 36 into the backing 38.

backing ring 38. As shown in FIG. 1, the second backing ring or dome 42 is then mounted on the shaft 28 and slid down into intimate physical contact with the opposite adjacent surface of ring 36, and the complete assembly is compactly and firmly pressed into an assembled unit by means such as a nut 44 which is threaded onto the end of the shaft into engagement .with the second backing ring or dome 42, preferably within a recess provided therefor, as illustrated.

The focal track ring or target ring 36 is thus firmly sandwiched between the two backing rings so that heat is efficiently transmitted from the ring 36 into the relatively massive bulks of the two backing rings.

Although the second backing ring or dome 42 is shown and described, there may be certain instances where this ring need not be provided, in which case the nut 44 is made to directly engage and exert pressure upon the target ring 36. Other desirable reasons for utilizing the second backing ring will be set forth, however, in a later part of this description.

From the above it will be apparent that when-the tube is operated a stream or beam of electrons will be emitted by the cathode 22 in the well-known manner and will impinge upon the adjacent inclined surface of the target ring 36, whereupon x-radiation will be emitted by this surface and will pass out of the tube through the x-ray transparent wall of the envelope 10. During this operation considerable heat is generated within the target ring 36. Therefore, to partially aid in the distribution of heat throughout the ring, as opposed to a localized area thereof, the target assembly 26 is caused to rotate at a relatively high speed so that a new surface area is constantly and continuously being presented to the electron beam, as is well known.

It was found that with a target assembly strucutred as described, under normal operating conditions the heat dissipation characteristics are greatly improved, with maximum temperatures of the three rings 36, 38 and 42 all being within about 750C to I,O40C during the tests performed, while with conventional target discs as presently made the temperature will approach about I,500C. No damage to the target results at the achieved lower temperatures because of the improved heat storage capacity and heat dissipation of the sandwich structure described. Because of the high emissivity and increased overall surface area the target assembly cools much more rapidly than known prior art targets, and these thermal improvements reduce the amount of heat flow to adjacent rotor bearings, thus also improving rotational performance and extending the life of the tube.

Referring now to FIG. 3, there is shown a rotating anode target assembly 26a which is similar to the target assembly shown in FIG. 1. However, in FIG. 3 the target ring 36a is sandwiched between backing rings 38a and 42a and are interconnected thereto by pins 46 as shown so that slippage between the respective rings is prevented. Furthermore, pins 46 can be adjusted by control of weight, size, location, etc. so as to provide means for dynamically balancing the target assembly. It will be noted that collar 40a is also similarly interconnected to backing ring 38a by pins 48. This will insure that the target assembly 26a will rotate withrotor shaft 28a Without slippage.

It has been found that a strong spring pressure will suitably retain the rings in assembled and intimate physical relation. One example of such a spring arrangement is shown in FIG. 4 wherein the target assembly 26b includes backing rings 42b and 3812 between which is sandwiched a target ring 36b. The rotor shaft 28b is provided with the aforementioned nut 44b which engages the second backing ring or dome 42b. However, instead of the previously described collar 40, this embodiment is provided with a spring device 50 of suitable shape which extends between backing ring 38b and the adjacent end of the rotor skirt 34!). Thus, the spring device constantly urges the three rings of the assembly into firm physical abutting relation so that efficient heat conduction is provided from target ring 36b into the backing rings.

In FIG. 5 there is shown a still further modification of arotating anode target assembly 260 embodying the invention. In this embodiment, the free end of the rotor shaft 28c is threaded to receive thereon a cup-shaped retainer 52 having an outwardly extending peripheral flange portion 54 which overlies and firmly engages a ledge or-shelf 56 provided therefor on the inner wall of the recess in backing ring or dome 420. In this embodiment the collar 40c engages the backing disc 380 while the flange 54 engages the dome 420. A nut 44c threaded onto shaft 28c then is moved into engagement with the base or bottom of the cup 52 as shown. Tightening of nut 44c will urge the three rings or discs of the target assembly into firm physical engagement with one another and cooperates with the flange 54 in retaining the assembled parts in such relationship.

In addition to the improved efficient heat transfer characteristics of this invention, an additional feature of importance is achieved by this invention. Referring to FIGS. 6 and 7, it will be seen that control of the size of the focal spot in one direction may be achieved by strict control of the width of the surface of the focal track which is exposed to the cathode. The focal track of target ring 36 is exposed throughout an annular surface area as shown and described, and it is upon this area that electrons in the form of a beam as indicated at 58 in FIG. 6 are directed from a filament 60 in cathode head 22. The cavity 62 within which the filament 60 resides is designed to perform some control of the size of the focal spot on the focal track which is impinged by the electron beam 58. However, such requires critical control and adjustment of the configuration of the recess 62 and the potentials applied to the filament 60 and to the head 22. In accordance with this invention, the width of the exposed focal track 36' is of a size which is intended to be the size desired of the focal spot length. Such a focal spot is indicated by the shaded area 64 in FIG. 7.

It is well known in the x-ray tube industry that it is desirable in most cases to provide a source of x-rays which emanates from a focal spot as small as possible. A conventional rotating target with its large surface area of relatively high atomic number is exposed to scattered secondary and primary high field emission electrons causing extrafocal radiation. This unwanted off focus radiation contributes to added patient dosage and degradation of radiographic image quality. Electron impingement, either primary or scattered, onto the low density elements 38 or 42 produce low energy xrays which are absorbed in the envelope 10. This off focus radiation is virtually eliminated except for that which occurs adjacent to the focal spot 64 on the target ring 36. This low energy radiation, however, is absorbed in the glass envelope of the tube and in other filtering material which may be placed in the x-ray beam. As shown in FIGS. 6 and 7,,a focal spot as viewed from the side of the tube desirably will appear as a substantially square spot as indicated by numeral 66.

A definite xray focal spot length, therefore, can be established by the appropriate selection of the width dimension of the exposed focal track as shown in F IGS. 6 and 7.'This feature precisely controls the focal spot length dimension even at high tube current levels where the electron beam tends to enlarge.

While the foregoing description has dealt with a composite target wherein the middle element of the sandwich is primarily of x-ray generating material, it is also possible to make the middle element, such as disc 36 in FIG. 1, of high thermal capacity material such as molybdenum or a mixture of about 95 percent molybdenum and about 5 percent tungsten, for example. In this case the focal track area will be comprised of a relatively thin layer of efficient x-ray generating material such as a mixture of about 90 percent tungsten and about 10 percent rhenium, for example. This is illustrated in FIG. 8 wherein disc 36d is sandwiched betubes such as, for example, the type shown in FIG. 8. The stationary anode tube shown in FIG. 8 includes an envelope 68 within one end portion of which is a cathode head 70 housing an electron emitting filament 72 which is intended to direct a beam of electrons toward an anode 74. Anode 74 is a body of copper, usually, which is provided with a hollow cylindrical extension portion 76 having an open end directed toward the cathode. An x-ray emitting target button 78 is provided in the base of the cavity thus formed in the anode for the purpose of receiving electrons from the cathode and directing resultant x-rays out through an opening 80 and then through the wall of the envelope 68.

In accordance with the present invention, a block or body of graphite or other selected backing material 82 is deposited in the bottom or the hollow anode extension and is provided with an inclined surface having a recess therein in which the target button 78 is positioned. A sleeve or shell 84 of graphite or other selected high thermal capacity material is then positioned in the extension with one end thereof engaging the tar which may contain a window 88 of beryllium or other material highly transmissive to xradiation which is suitably aligned with opening 80 in extension 76 whereby x-rays emanating from the target button 78 will pass outwardly through the window 88 and opening 80.

It is, of course, necessary that there be efficient conduction of heat from the target button 78 into the adjacent graphite bodies 82 and 84. Therefore, means is provided for continually urging the sleeve 84 against the target button 78 and to thereby maintain efficient heat conductive relationship of the button 78 with backing 82. Such means is illustrated in exemplary form as a spring 90 which at one end engagesthe outer end of the shell 84 and at its other end engages the inner side of a retaining ring or collar 92 which is attached to the inner circumference of the anode extension 78 as by set screws 94 or the like.

Thus, there is described one type of stationary anode tube which has most, if not all, of the advantages of the rotating anode structure described hereinbefore. Other stationary anode tubes may be provided with this invention, however, such as the type which embodies a metal housing without the glass envelope, as is well known.

In any of the aforementioned embodiments of the invention thermal expansion between the target member and the high thermal conductivity backing elements is permitted. This, then, opens up the possiblity ofa large number of materials which may be used to serve the desired functions. Low density elements can serve the function of the target backings while selected different materials may be used for the x-ray producing elements of the structures.

From the foregoing description it will be apparent that all of the objects and advantages of this invention have been achieved by the various structures shown and described. It will be apparent, however, that various modifications may be made by those skilled in the art without departing from the spirit of the invention as expressed in the accompanying claims. Therefore, all matter shown and described should be interpreted as illustrative and not in a limiting sense.

I claim:

1. An anode for X-ray tubes having a cathode which operates to produce an electron beam of known crossget button 78. Sleeve 84 is provided with an opening 86 v sectional dimensions, said anode including a target assembly comprising a target member of material having a known thermal storage capacity per unit weight and capable of X-ray emission when impinged by said electron beam, a pair of supporting members one on each side of the target member, and means for engaging said supporting members to sandwich the target member therebetween, said supporting members overlying the target member in a manner whereby only a selected surface area of the target member is exposed and both having substantially higher thermal storage capacity per unit weight than theflmaterial of said target member, said surface area being of a size in one direction which corresponds to one cross-sectional dimension of said electron beam impinging thereon, and said supporting members comprising means for preventing substantial production of off-focus radiation.

2. An anode for X-ray tubes as set forth in claim 1 wherein said supporting members are disclike in shape, said target member is annular in shape, and said focal area is an annular surface portion of the target member.

3. An X-ray tube comprising ahermetically sealed envelope, a cathode electrode and an anode electrode located in spaced relation within the envelope, and means for connecting said electrodes to external sources of electrical energy, said cathode electrode comprising means for producing an electron beam of known cross-sectional dimensions, said anode ,electrode including a target'assembly comprising a target member of material having a known thermal storage capacity per unit weight and capable of emission of X- rays and secondary electrons when impinged by said electron beam, a pair of supporting members one on each side of the target member, and means for engaging said supporting members to sandwich the target member therebetween, said supporting members overlying the target member in a manner whereby only a selected surface area of the target member is exposed and both having substantially, higher thermal storage capacity per unit weight than the material of said target memher, said supporting members further being of a material incapable of substantial production of X-rays when impinged by secondary electrons from said target, said exposed surface area being of a size in one direction which corresponds to one cross-sectional dimension of said electron beam impinging thereon.

4. An X-ray tube as set forth in claim 3 wherein said supporting members are disclike in shape, said target member is annular in shape, and said focal area is an annular surface portion of the target member.

5. A rotating anode X-ray tube comprising a hermetically sealed envelope, a cathode electrode and an anode electrode located in spaced relation within the envelope, and means for connecting said electrodes to external sources of electrical energy, said cathode electrode comprising means for producing anelectron beam of known cross-sectional dimensions, said anode electrode comprising a rotatable shaft and a target assembly mounted on the shaft for rotation therewith, said target assembly. comprising a target member of material having a known thermal storage capacity per unit weight and capable of emission of X-rays and secondary electrons when impinged by said electron beam, a pair of supporting members one on each side of the target member, both of which members have a thermal storage capacity per unit weight which is substantially higher than the target member, and means for engaging said supporting members to sandwich the target member therebetween, said supporting members overlying the target member in a manner whereby only a selected surface area of the target member is exposed and having substantially higher thermal storage capacity per unit weight than the material of said target member, said supporting members further being of a material incapable of substantial production of X-rays when impinged by secondary electrons from said target, said surface area being of a size in one direction which corresponds to one cross-sectionaldimension of said electron beam impinging thereon,

6. A target assembly for X-ray tube anodes comprising a target member and a pair of supporting members located one on each side of the target member, said supporting members overlying the target member except for an elongated focal track thereon, said focal track being of a size in the lateral direction which corresponds to one dimension of a desired X-ray generating focal spot to be produced thereon, and said supporting members being of material incapable of production of substantial amounts of X-radiation when impinged by secondary electrons from said focal track.

7. An-X-ray tube comprisinga hermetically sealed envelope, a cathode electrode and an anode electrode located in spaced relation within the envelope, and means for connecting said electrodes to external sources of electrical energy, said anode comprising a support, and an X-ray generating target assembly mounted on the support, said target assembly comprising a target member having a focal track on the side thereof facing the cathode electrode of a material capable of emission of X-rays and secondary electrons when impinged by electrons from said cathode electrode, and a pair of supporting members one on each side of the target member, said supporting members overlying the surface of the target member facing said cathode electrode with only said focal track being exposed, said exposed focal track being of a size in the lateral direction which corresponds to one dimension of a desired X-ray generating focal spot to be produced thereon, and said supporting members being of a material incapable of substantial production of X-radiation when impinged by secondary electrons from said focal track.

8. An X-ray tube as set forth in claim 7 wherein said supporting members are disc-like in shape, said target member is annular in shape, and said focal track is an annular surface portion of the target member. 

1. An anode for X-ray tubes having a cathode which operates to produce an electron beam of known cross-sectional dimensions, said anode including a target assembly comprising a target member of material having a known thermal storage capacity per unit weight and capable of X-ray emission when impinged by said electron beam, a pair of supporting members one on each side of the target member, and means for engaging said supporting members to sandwich the target member therebetween, said supporting members overlying the target member in a manner whereby only a selected surface area of the target member is exposed and both having substantially higher thermal storage capacity per unit weight than the material of said target member, said surface area being of a size in one direction which corresponds to one crosssectional dimension of said electron beam impinging thereon, and said supporting members comprising means for preventing substantial production of off-focus radiation.
 2. An anode for X-ray tubes as set forth in claim 1 wherein said supporting members are disclike in shape, said target member is annular in shape, and said focal area is an annular surface portion of the target member.
 3. An X-ray tube comprising a hermetically sealed envelope, a cathode electrode and an anode electrode located in spaced relation within the envelope, and means for connecting said electrodes to external sources of electrical energy, said cathode electrode comprising means for producing an electron beam of known cross-sectional dimensions, said anode electrode including a target assembly comprising a target member of material having a known thermal storage capacity per unit weight and capable of emission of X-rays and secondary electrons when impinged by said electron beam, a pair of supporting members one on each side of the target member, and means for engaging said supporting members to sandwich the target member therebetween, said supporting members overlying the target member in a manner whereby only a selected surface area of the target member is exposed and both having substantially higher thermal storage capacity per unit weight than the material of said target member, said supporting members further being of a material incapable of substantial production of X-rays when impinged by secondary electrons from said target, said exposed surface area being of a size in one direction which corresponds to one cross-sectional dimension of said electron beam impinging thereon.
 4. An X-ray tube as set forth in claim 3 wherein said supporting members are disclike in shape, said target member is annular in shape, and said focal area is an annular surface portion of the target member.
 5. A rotating anode X-ray tube comprising a hermetically sealed envelope, a cathode electrode and an anode electrode located in spaced relation within the envelope, and means for connecting said electrodes to external sources of electrical energy, said cathode electrode comprising means for producing an electron beam of known cross-sectional dimensions, said anode electrode comprising a rotatable shaft and a target assembly mounted on the shaft for rotation therewith, said target assembly comprising a target member of material having a known thermal storage capacity per unit weight and capable of emission of X-rays and secondary electrons when impinged by said electron beam, a pair of supporting members one on each side of the target member, both of which members have a thermal storage capacity per unit weight which is substantially higher than the target member, and means for engaging said supporting members to sandwich the target member therebetween, said supporting members overlying the target member in a manner whereby only a selected surface area of the target member is exposed and having substantially higher thermal storage capacity per unit weight than the material of said target member, said supporting members further being of a material incapable of substantial production of X-rays when impinged by secondary electrons from said target, said surface area being of a size in one direction which corresponds to one cross-sectional dimension of said electron beam impinging thereon.
 6. A target assembly for X-ray tube anodes comprising a target member and a pair of supporting members located one on each side of the target member, said supporting members overlying the target member except for an elongated focal track thereon, said focal track being of a size in the lateral direction which corresponds to one dimension of a desired X-ray generating focal spot to be produced thereon, and said supporting members being of material incapable of production of substantial amounts of X-radiation when impinged by secondary electrons from said focal track.
 7. An X-ray tube comprising a hermetically sealed envelope, a cathode electrode and an anode electrode located in spaced relation within the envelope, and means for connecting said electrodes to external sources of electrical energy, said anode comprising a support, and an X-ray generating target assembly mounted on the support, said target assembly comprising a target member having a focal track on the side thereof facing the cathode electrode of a material capable of emission of X-rays and secondary electrons when impinged by electrons from said cathode electrode, and a pair of supporting members one on each side of the target member, said supporting members overlying the surface of the target member facing said cathode electrode with only said focal track being exposed, said exposed focal track being of a size in the lateral direction which corresponds to one dimension of a desired X-ray generating focal spot to be produced thereon, and said supporting members being of a material incapable of substantial production of X-radiation when impinged by secondary electrons from said focal track.
 8. An X-ray tube as set forth in claim 7 wherein said supporting members are disc-like in shape, said target member is annular in shape, and said focal track is an annular surface portion of the target member. 